Kook joined the pre-state Haganah militia in 1930 following widespread Arab riots. In 1931, Kook helped found the Irgun, a group of militant Haganah dissidents, and fought with them in Palestine through most of the 1930s. He served as a post commander in 1936, and eventually became a member of the Irgun General Headquarters.

In 1937 Kook began his career as an international spokesperson for the Irgun and Revisionist Zionism. He first went to Poland, where he was involved in fundraising and establishing Irgun cells in Eastern Europe. It was there that he met the founder of the Revisionist movement, Ze'ev Jabotinsky, and became friends with his son Ari. At the founders' request, Kook traveled to the United States with Jabotinsky in 1940,[1] where he soon served as the head of the Irgun and revisionist mission in America, following the elder's death in August. This assignment was clandestine, and Kook publicly denied he was affiliated with the Irgun many times while in America.

Initially the Bergson Group largely limited its activities to Irgun fundraising and various propaganda campaigns. The outbreak of World War II saw a dramatic transformation in the group's focus. As information about the Holocaust began to reach the United States, Kook and his fellow activists became more involved in trying to raise awareness about the fate of the Jews in Europe. This included putting full-page advertisements in leading newspapers, such as "Jews Fight for the Right to Fight", published in The New York Times in 1942, and "For Sale to Humanity 70,000 Jews, Guaranteed Human Beings at $50 a Piece", in response to an offer by Romania to send their Jews to safety if the travel expenses would be provided. On March 9, 1943, the Group produced a huge pageant in Madison Square Garden written by Ben Hecht, titled "We Will Never Die", memorializing the 2,000,000 European Jews who had already been murdered. Forty thousand people saw the pageant that first night, and it went on to play in five other major cities including Washington, D.C., where First Lady Eleanor Roosevelt, six Supreme Court Justices, and some 300 senators and congressmen watched it.

In 1943, Kook established an "Emergency Committee for the Rescue of European Jewry". The Committee, which included Jewish and non-Jewish American writers, public figures, and politicians, worked to disseminate information to the general public, and also lobbied the President and Congress to take immediate action to save the remnants of Europe's Jews. United States immigration laws at the time limited immigration to only 2% of the number of each nationality present in the United States since the census of 1890, which limited Jews from Austria and Germany to 27,370 and from Poland to 6,542; even these quotas often went unfilled, due to United States State Department pressure on US consulates to place as many obstacles as possible in the path of refugees.

The proposal to admit more refugees was ratified by the United States Senate Committee on Foreign Relations, and, in response to the pressure, President Roosevelt subsequently issued an administrative order for the establishment of a special national authority, the War Refugee Board (WRB) to deal with Jewish and non-Jewish war refugees. An official government emissary sent to Turkey was of considerable assistance in the rescue of Romanian Jewry. The WRB saved about 200,000 Jews.[2] Those rescued through the WRB were probably mostly in Hungary, in part through the Raoul Wallenberg mission which was sponsored by the WRB.

Kook and his followers were widely opposed by American Zionist organizations. In December 1943, the American Jewish Conference launched a public attack against the Bergsonites in an attempt to derail support for the resolution. [3] The British embassy and several American Zionist groups, including the American Jewish Committee and other political opponents sought to have Kook deported or drafted. [4] They encouraged the IRS to investigate the Bergson Group's finances in an attempt to discredit them, hoping to find misappropriation, or at least careless bookkeeping, of the large amount of funds the groups handled. The United States IRS found no financial irregularities[5] Among those trying to stop the Bergson Group's rescue activities were Steven Wise, Nahum Goldmann and Congressman Blum. A State Department protocol shows Goldmann telling the State Department that Hillel Kook didn't represent organized Jewry, and suggested either deporting him or drafting him for the war effort. [6]

Joined by Bergson Group activists, the Jewish War Veterans of the United States of America marched on the United States Capitol, Lincoln Memorial, and White House in Washington, DC. They were met by a number of prominent members of Congress including William Warren Barbour, the protesters plead for U.S. intervention on behalf of the Jews in Europe. Though the delegation was reluctantly received by Vice PresidentHenry Wallace, Franklin Roosevelt avoided them entirely, both out of concerns regarding diplomatic neutrality, but also because he was influenced by the advice of some of his Jewish aides and several prominent American Jewish spokespeople (including Dr. Stephen Wise who told him "Mr. President, you wouldn't want to be associated with these people"), who thought the protest would stir up anti-Semitism and claimed that the marchers, many of whom were both Orthodox as well as recent immigrants (or first-generation Americans) were not representative of American Jewry. Shortly before the protest reached the White House, FDR left the building through a rear exit to attend a Army ceremony, and then left for a weekend in the country.

Disappointed and angered by the President's failure to meet with them, the rabbis stood in front of the US Capitol, where they were met by Senator William Barbour and other members of Congress, and refused to read their petition aloud, instead handing it off to the Presidential secretary, Marvin H. McIntyre. The march garnered much media attention, much of it focused on what was seen as the cold and insulting dismissal of many important community leaders, as well as the people in Europe they were fighting for. One Jewish newspaper commented, "Would a similar delegation of 500 Catholic priests have been thus treated?"[8] Years later, Rabbi Soloveitchik, in recorded lectures, would bemoan the betrayal of the Rabbis' mission by Stephen Wise, who dismissed them as a group of Orthodox rabbis who didn't represent anyone.[9] A week later, Senator William Warren Barbour (R; New Jersey), one of a handful of politicians who met with the rabbis on the steps of the US Capitol, proposed legislation that would have allowed as many as 100,000 victims of the Holocaust to emigrate temporarily to the United States. A parallel bill was introduced in the House of Representatives by Rep. Samuel Dickstein (D; New York). This also failed to pass.

Without waiting for the end of World War II, the Irgun declared an open revolt against British rule in Palestine.[10] To assist in recruiting and propaganda efforts, Kook established the Hebrew Committee for National Liberation and the American League for a Free Palestine, both of which were involved in lobbying U.S. and other diplomats and in trying to attract the American public to support the Irgun's rebellion. Kook remained strongly affiliated with the Revisionist camp after the war during the creation of the state of Israel. While he was unquestionably loyal to the cause, his position as the Irgun's leading American activist was not free from conflict. In 1946 Kook received a letter from Menachem Begin, who had become chief of the Irgun in 1943. Begin admonished Kook for various policy positions that strayed from the official Irgun party-line. These included Kook focusing on the transportation of illegal immigrants to Palestine instead of a "primary" assignment - arms shipments to Irgun fighters, as well as a (rather common) usage of the term "Palestine". At the time Kook was in the habit of saying "Palestine Free State", which Begin thought left too much potential for bi-nationalism. Begin demanded that instead that Kook in public refer to the future Jewish state as the "Free State of Eretz Israel". Begin criticized Kook for keeping too high a profile, angrily reminding him that the Irgun was an underground organization; he was supposed to be using his resources to help the revolt in Palestine, not organizing parades and marches.

Begin's letter illustrated the deep tensions that existed between the Irgun leadership and its independent but influential activists in the United States. It also revealed the increasing ideological schism that emerged between the Begin's camp, which inherited the political and military infrastructure of Jabotsinky, and Kook's followers, who saw themselves as Jabotinsky's true ideological and political heirs. This tension would later come to a head when Kook and many of the Bergson Group members returned to the new State of Israel in 1948.

In 1947, the Bergson Group had purchased a ship originally intended to carry new immigrants to Palestine, but, perhaps partially due to Begin's influence, was eventually used to ship arms. The ship was named Altalena, and was the focus of a violent confrontation between the newly formed Israel Defense Forces and the Irgun on the beaches of Kfar Vitkin and Tel Aviv. Following the Altalena Affair, Kook was arrested with four other senior Irgun commanders and held for over two months. Of the five, only Kook was a member of the Bergson Group. The five were eventually released after about two months.

Kook served in the first Knesset as part of the Herut party list, but quit the party with his close friend and fellow Herut Member of the Knesset Ari Jabotinsky. This followed two years of ongoing disagreements with their colleagues, particularly Menachem Begin, over the party's leadership and direction. Kook, who had returned to Israel after a ten-year absence, was now confronted with the reality that the country and movement he had fought for bore little resemblance to his ideals. Kook and Jabotinsky served as independent or "single" MKs for the remaining months of their terms, the first ever to do so. Profoundly disillusioned with the Israeli political process and future of the Revisionist movement, Kook left Israel in 1951 with his wife and daughter. In 1968, four years after his wife's death, he returned to Israel with his two daughters. He remarried in 1975 and lived near Tel Aviv until his death in 2001.

While Kook never re-entered politics, he continued to give interviews for many years, in which he continued to articulate his independent perspectives on Zionism, Jewish identity, and Israeli politics. His more controversial ideas included declaring that Jabotinsky's primary goal in creating a Jewish state was in making a country to which all Jews would want to belong, and that once Israel had been created, any Jews who refused to make aliyah had made a conscious choice to become "integrated" citizens of their naturalized countries. This distinction between Jews and Hebrews was another major sticking point between Kook and the larger Irgun leadership as early as the mid–1940s. Kook's views in this area can be seen as a more moderate version of the "Canaanist" ideology espoused by Yonatan Ratosh.[11][12] Like Ratosh, Kook was influenced by Adolf Gurevich, a Betar activist with connections to Bergson Group members Shmuel Merlin and Eri Jabotinsky.[12][13]

Kook had a specific body of critiques concerning what he saw as the distortion of Zionist philosophy and idealism by Israeli politics. He maintained that he had always conceived of Israel being a "Jewish state" by having a majority of Jewish citizens, not through specific associations to Jewish nationalism. Paradoxically, Kook's "theocratic" vision of Israel gave him a great deal of ideological flexibility in regards to some of Israel's more intractable problems. He supported accordingly, all non-Jewish citizens of Israel with full rights and privileges, and once, in an interview with an Israeli Druze, commented that, like Jabotinsky, he saw "no reason" why the State of Israel could not have a non-Jewish president. He was in favor of amending the Law of Return to consider prospective immigrants on an individual, and not national or religious basis, except for cases of immediate danger.

Kook was also a strong supporter of Israel's constitution, which had been stalled during its writing in 1948 and never completed. Kook claimed that a formal constitution could have solved many ongoing issues in Israeli society, such as discrimination against Israeli Arabs, by providing all of Israel's citizens with a clearly defined, and egalitarian, role in Israeli nationalism. He once remarked that the lack of a constitution was "Israel's greatest tragedy": that Ben-Gurion's decision to change the Israeli governing body from a Constituent Assembly to a Parliament had been a putsch, and that he regretted not having resigned from the Knesset immediately after the decision had been made. Kook also favored the creation of a Palestinian state, albeit one established in modern-day Jordan. He was one of the first Israelis to call for a Palestinian state shortly after the Six-Day War. For the remainder of his life, Kook adamantly claimed that his position would have been shared by his mentor Jabotinsky.

Kook repeatedly referred to himself as a post-Zionist, and was one of the first in Israeli society to voluntarily (and positively) adopt the term.

Since the late 1990s, some historians have attempted to re-examine and evaluate the significance of his activities during World War Two and his role as a political opponent of Begin. One allegation is that Kook's adversaries in Israel and America downplayed some of his accomplishments and minimized their own role in curtailing his activities. David Wyman and Rafael Medoff, co-authors of a 2002 Kook biography,[14] suggested that, had it not been for the interference of the American Jewish establishment, Kook might have become as successful a rescuer as Oskar Schindler or Raoul Wallenberg. A play, The Accomplices, written by Bernard Weinraub and based on Kook's wartime efforts in the United States premiered at The New Group in 2007 and played thereafter in regional theatres.[15][16] It played also in Jerusalem in April 2009.

The role of Hillel Kook (aka "Peter Bergson") was played twice onstage by actor Steven Schub (lead singer of The Fenwicks), in 2008 at The Fountain Theatre and in 2009 at the Odyssey Theatre in Los Angeles. Actor Raphael (Rafi) Poch (Artistic Director of J-Town Playhouse) played Hillel Kook in Jerusalem.

We, the Hebrews, descendants of the ancient Hebrew nation, who remained alive on God's earth despite that great calamity that our people have experienced, have come together in the Hebrew Committee of National Liberation. The Jews today who live in the European hell together with the Jews in the Land of Israel constitute the Hebrew nation—there isn't another nation to which they owe their allegiance but the Hebrew nation. We must state it clearly: the Jews in the United States do not belong to the Hebrew nation. These Jews are Americans of Hebrew descent. - From A Manifesto of the Hebrew Nation, 1944.

Why did we respond the way we did? The question should be, why didn't the others? We responded as a human and as a Jew should. - On his Holocaust activism, 1973.

I, who was the liaison officer of the Irgun central command with Jabotinsky, and who accompanied him almost daily for four years—remained loyal to his teachings. I also believe that the Land of Israel, on both banks of the Jordan River, is our historic homeland. But I am also certain that had Jabotinsky lived today, he would have argued that now, after we've achieved our independence, our mission is to attain peace in order to establish the Israeli people as the political heir of the Jewish people.- Interview in 1977.

There is no exile. The exile ended on May 14, 1948. - Interview in 1982.